Exercise spring



FIG. 1 is a perspective view of a exercise spring showing my new design;

FIG. 2 is a top view with the bottom view being a mirror image thereon;

FIG. 3 is a rear view;

FIG. 4 is a left end view;

FIG. 5 is a front view; and,

FIG. 6 is a right end view. 

The ornamental design for an exercise spring, as shown and described. 